1. Field of the Invention
This invention relates to heating systems including furnaces having fuel-fired burners, and, more particularly, to a control arrangement for motor actuated stack dampers and fuel supply apparatus for such systems.
2. Description of the Prior Art
Heating systems employing furnaces having fuel-fired burners require a vent stack to conduct combustion products away from the burner. Automatically controlled stack dampers are generally used in the ventilation stacks to permit the stacks to be closed when the furnace is not operating to minimize heat losses when the furnace is not operating. However, for safe operation, it is necessary that the stack damper be open in advance of each operation of the burner and that the damper be maintained open for a short time following each operation of the burner to allow volatiles to be purged from the furnace following each operation. Accordingly, systems in which automatic dampers are used generally include a control arrangement which provides an interlock between the damper control mechanism and fuel supply apparatus of the system to assure that the damper is fully open before the burner operates and is maintained open for a short time after the completion of the operation of the burner.
One such arrangement is disclosed in the U.S. Pat. No. 3,010,451 to Hodgins in which a primary burner control is conditional on and subsequent to the opening of the damper. A damper drive motor is energized in response to a call for heat to drive the damper to an open position. A normally open microswitch, which is connected in series with the burner control circuit, is operated to complete the burner circuit when the damper reaches the fully open position. At the end of the run, the damper drive motor is deenergized, and a bias spring permits the damper to return to the closed position. Movement of the damper from the fully open position permits the microswitch to open, interrupting the burner circuit. A time lag is provided between the interruption of the burner circuit and the closing of the damper to allow volatiles to be purged from the furnace following operation of the burner.
One common feature of known prior art systems in which control circuits for the burner are interlocked with damper control circuits is that the dampers are driven to and maintained in the open position through use of a motor or solenoid and returned to a closed position through the action of a spring bias mechanism when the motor or solenoid is deenergized. In such systems, the motor or solenoid requires continuous energization when the burner is operating to maintain the damper in the open position, requiring continual power usage while the furnace is operating. Also, in the event of a momentary power loss, the damper is automatically returned to a fully closed position, and upon restoration of power, the damper must be driven to the open position before a heating cycle can be initiated.
Moreover, a safety standard recently established by the American Gas Association requires that stack dampers be biased in such a manner that the damper returns to a fully open position upon the loss of power. Accordingly, modification of known damper control units to meet such standard would require that the drive motor or solenoid be energized in a stalled condition whenever the furnace is off. Such requirement not only results in large power losses, but also decreases the lifetime of the motor or drive solenoid.
Therefore, it would be desirable to provide an automatic damper control arrangement for use in a heating system which minimizes power consumption. It would also be desirable to have a motor driven damper apparatus for use in a heating system which is interlocked with fuel supply apparatus of the system, and which prevents operation of the fuel supply apparatus whenever the damper is in a position other than a fully open position.